1. Field of the Invention
The invention relates to an electric power steering apparatus which is provided with an electric motor and which assists a turning operation of a driver's steering wheel performed by a driver.
2. Description of the Related Art
There is an electric power steering apparatus for a vehicle that detects a steering torque applied to a steering wheel which is turned by a driver (hereinafter, referred to as a “driver's steering wheel”) and that causes an electric motor to produce an assist torque based on the detected steering torque. For example, in an electric power steering apparatus described in Japanese Patent Application Publication No. JP-2003-26020 (JP-A-2003-26020), a controller calculates a target current value for an electric motor based on a steering torque applied to a driver's steering wheel and a vehicle speed, and the controller calculates a target value of the voltage at which electricity is supplied to the electric motor based on the deviation of a motor current value (actual current value) detected by a current detector from the target current value. Then, the controller executes the switching control on an inverter at a duty ratio corresponding to the target voltage value, whereby a target three-phase power-supply voltage is applied from the inverter to the electric motor. As a result, a desired assist torque is produced.
For example, a three-phase permanent magnet motor is used as an electric motor of an electric power steering apparatus. When the rotation of the electric motor is controlled by a three-phase power supply unit, the vector control indicated by the two-phase rotating flux coordinate system (the d-q coordinate system) is usually executed. When the vector control is executed, the rotational angle (the electric rotational angle position of a rotor) is detected, and the two-phase/three-phase coordinate conversion (the coordinate conversion from two-phase to three-phase, and the coordinate conversion from three-phase to two-phase) is executed based on the rotational angle.
Accordingly, if a malfunction occurs in a rotational angle sensor that detects the rotational angle of the electric motor, it becomes difficult to execute the control. According to Japanese Patent Application Publication No. JP-2003-26020 (JP-A-2003-26020), a resolver sensor is used as a rotational angle sensor. When the output (the amplitude value of a periodic waveform signal) from the resolver sensor is equal to or lower than a predetermined value due to, for example, breaking of wire, it is determined that a malfunction has occurred in the resolver sensor. According to JP-A-2003-26020, even after it is determined that a malfunction has occurred in the resolver sensor, the assist control is continuously executed to produce the assist torque when the electric motor is at a rotational angle position at which the output from the resolver sensor is equal to or higher than a predetermined level. As a result, the situation where the driver suddenly falls into difficulty in the steering operation is avoided.
However, executing the control based on a detection signal from such malfunctioning rotational angle sensor is not as reliable as it should be. When a salient-pole permanent magnet motor is used as the electric motor that produces a steering assist torque, the rotational angle is estimated based on the salient-polarity (a change in the magnetic resistance due to a change in the position of a rotor). Accordingly, it is considered that the rotational angle of the electric motor is estimated without using a sensor, and the assist control is continuously executed by controlling the amount of electricity supplied to the electric motor based on the estimated rotational angle.
In an electric power steering apparatus, however, an electric motor is rotated at a considerably low speed that is close to 0 in many cases, unlike a common electric motor. If the electric motor is rotated at such a low speed, the reliability of the estimated value of the motor rotational angle is low. Namely, as the motor speed (the rotational angular speed) of the electric motor decreases, the reliability of the estimated value of the rotational angle obtained using the salient-polarity of the motor is reduced. Accordingly, if the electric motor of the electric power steering apparatus is driven under the sensorless control without taking any measures, the target assist torque is not achieved. In some cases, a reverse assist torque may be produced, that is, an assist torque may be undesirably applied in the direction opposite to the direction in which the target assist torque should be applied.